This application is related to my copending applications 849,271 entitled "High-Speed FFT Processor" and 849,279 entitled "A Configurable Parallel Arithmetic Structure For Recursive Digital Filtering," filed concurrently herewith and assigned to the same assignee as the present invention.
This invention is generally related to the field of communications and, more particularly, to the field of satellite communications. The invention disclosed is specifically directed to an apparatus for interfacing conventional FDM signals with the increasingly used time division multiplexed-pulse code modulated (TDM-PCM) signals in satellite communications systems.
Voice transmissions constitute a large portion of international satellite communications. The voice channels are formed into a set of single-side band (SSB) suppressed carrier signals which are frequency division multiplexed (FDM) at 4 kHz spacing on a multi-carrier wide band analog sub-carrier. Traditionally, these sub-carriers may carry 12 voice channels (group), 60 voice channels (super group) and 300 voice channels (master group). In recent years, however, TDM-PCM voice band signal transmission has been used in some telephony systems. In this latter mode of transmission, digital PCM signals are interleaved in real time to form a single-bit stream. Since digital technology is expected to continue to progress at a faster rate than analog technology, it is generally agreed that more and more transmission systems will be implemented digitally as time progresses. Because so many analog FDM transmission systems already exist, it is expected that it may be several decades before most of the transmission systems become digital. During this time, some practical means of high-level multiplexed signal conversion is required to facilitate interconnection of the two types of transmission systems.
Direct PCM encoding of a 60-channel super group in which each channel has a band width of 4 kHz requires a Nyquist sampling rate of 8 kHz for each of the 60 channels. At 8 bits/sample, the information processing speed would be (8 kilo-samples/channel-second).times.(8 bits/sample).times.(60 channels)=3.840 M bits/sec. Digital speech interpolation (DSI) is capable of reducing the required transmitted bit rate by one half, and other encoding techniques, such as delta-modulation, may offer further reduction. These bit rate reduction techniques require that the super group be demultiplexed into separate channels at one end and multiplexed back into a super group at the other end.
To perform demultiplexing and multiplexing of an analog FDM super group would require 60 high-order crystal filters, 60 mixers and 60 heterodyning oscillators. Such equipment occupies a relatively large area and significantly increases the size requirements of earth stations. For example, the above-mentioned equipment occupies approximately 12 standard 6-foot.times.31/2-foot.times.2-foot racks. By 1980, it is expected that traffic requirements will increase significantly; therefore, there is a need for a reduction in equipment size in order to reduce the size requirements of the earth stations. It is generally desirable to design a FDM/TDM transmultiplexer in which as much of the signal processing as possible is performed digitally in order to reduce the size, complexity and cost of the equipment while enabling higher operating speeds. Several transmultiplexers have been designed which take advantage of recent advances in digital signal processing.
One such device is described by Takahata, et al., "Development of TDM/FDM Transmultiplexer," which was to have been delivered at the IEEE National Communications Conference, Dec. 5, 1977. In that device, TDM to FDM conversion is achieved by first converting the 8-bit A- or .mu.-law PCM signals into 18-bit linear code and then reversing the frequency spectra of odd-numbered channels. The format-converted PCM signals are fed to a N.times.N FFT processor, and the 2N complex FFT outputs are multiplied by phase offsets. The real parts of the products of the multiplication are filtered by a bank of 2N digital non-recursive filters, and the outputs of the filters are delayed, combined and converted to analog form, resulting in a SSB FDM signal. For FDM and TDM conversion, the process is reversed. The Takahata, et al. system is unsatisfactory since it is only capable of handling 12-channel groups rather than 60-channel super groups. In order to expand the Takahata, et al. system to accommodate super groups, a bank of 120 digital real sub-filters would be required in addition to 120 delay elements and phase offset circuits.
The present invention provides a more effective transmultiplexer which, in TDM and FDM conversion, selects nonoverlapping side bands from the frequency spectra of each TDM channel and combines the channel signals to form an FDM signal. In order to achieve the required filtering, each channel is passed through a digital filter to remove every other side band in its frequency spectra. In order to conserve as much space and expense as possible, and to achieve a modular transmultiplexer design for simplified maintenance, a digital filter is preferred which can be multiplexed over all channels so that a bank of filters would not be required.